The proposed research will investigate possible physiological audiometric tests based on the auditory-nerve action potential (AP) and brainstem-evoked response (BSER). APs and BSERs will be recorded from external auditory meatus and vertex, respectively, using non-traumatic electrodes and digital averaging. A proposed test to approximate the pure-tone audiogram will utilize filtered click AP threshold elevations relative to normal. Test accuracy will be increased by using the derived AP technique to plot basilar-membrane excitation patterns of filtered clicks and adjusting frequencies assigned to the APs according to these plots. To develop a test to localize auditory pathology, AP and BSER input-output data from selected patients with localizable hearing loss will be investigated for consistent localizing signs. A pilot study suggests that such data will yield more consistent localizing signs if the common practice of adding condensation and rarefaction responses is not followed. A parallel study will investigate derived click AP input-output data for localizing signs. The proposed research will also investigate a "physiological critical band" demonstrated in a pilot study. The physiological critical band will be compared to the subjective critical band by comparing physiological critical bandwidths across intensities and frequencies and also by comparing physiological and subjective critical bandwidths in the same subject using the same stimuli. We will also investigate the physiological critical band in hearing-loss subjects for possible localizing and quantifying signs.